The present invention relates generally to a method and system for controlling air/fuel ratio in an internal combustion engine. More particularly, the invention relates to a method and system for adjusting the air/fuel ratio of an internal combustion engine during a time period when a hydrocarbon trap, which is coupled to the engine, is being purged.
Certain automotive vehicles are equipped with emission control devices, commonly referred to as hydrocarbon (HC) traps, that adsorb hydrocarbons when the temperature of the device is below a certain level and release and oxidize the stored hydrocarbons when the temperature of the device rises above a certain level. HC traps are particularly useful in a vehicle""s exhaust system in combination with a three-way catalytic converter (a xe2x80x9cTWCxe2x80x9d) positioned upstream of the HC trap. In steady-state operation, conventional three-way catalysts store oxidants (NOx and oxygen) when the engine is operated with a lean air/fuel ratio and release the oxidants when the engine is operated with a rich air/fuel ratio. The released oxidants react with the incoming HC and CO (produced when the engine is operated with a rich air/fuel ratio) to form H2O and CO2. In this way, HC and CO are oxidized and NOx is reduced. However, conventional three-way catalysts are relatively ineffective below a certain temperature. Therefore, HC traps are sometimes used in the same exhaust system with three-way catalysts to store the HC produced by the engine during and after initial start-up and prior to the three-way catalyst reaching a temperature at which it can effectively reduce NOx and oxidize HC and CO.
When the temperature of an HC trap reaches a certain purge threshold temperature, the HC trap begins to release the HC that it stored when the temperature of the HC trap was relatively low. As with three-way catalysts, the released HC reacts with oxygen in the exhaust stream to form H2O and CO2. To minimize the amount of unreacted HC that is emitted into the atmosphere, it is important to ensure that there is sufficient oxygen present in the exhaust stream entering the HC trap to oxidize as much of the released HC as possible. As described herein, one method to ensure that sufficient oxygen reaches the HC trap is to use an air pump to pump air into the exhaust stream just upstream of the HC trap.
Three-way catalysts are generally known to reduce NOx more efficiently when the engine is operated with an air/fuel ratio that is rich of stoichiometry. This is because HC and CO are produced by the engine when it is operated with a rich air/fuel ratio, and the HC and CO that enter the three-way catalyst re act with and reduce the stored NOx. Therefore, for purposes of NOx reduction, it is generally desirable to operate the engine somewhat rich of stoichiometry. However, engine control strategies do not typically operate the engine with a rich air/fuel mixture for an extended period of time for various reasons, including the fact that the excessive HC and CO production and emissions would be undesirable. In particular, while abundant HC and CO in the exhaust stream (due to extended rich engine operation) would virtually eliminate NOx emissions, there would not typically be sufficient oxygen in the exhaust stream to oxidize the HC and CO.
The inventors have recognized that it is desirable to utilize engine control strategies t ha t enable the three-way catalyst to efficiently reduce Nox while, at the same time, avoid the risk of increased hydrocarbon emissions into the atmosphere.
The present invention is directed to an improved method and system for controlling the air/fuel ratio supplied to an internal combustion engine that more efficiently reduces NOx emissions during the purging of an HC trap without increasing HC and CO breakthrough. An internal combustion engine is coupled to an exhaust system that includes a three-way catalytic converter positioned downstream of the engine. A hydrocarbon (HC) trap is positioned in the exhaust stream downstream of the three-way catalyst. An air pump is connected to the exhaust stream between the three-way catalyst and the HC trap and is capable of selectively providing air mass to the exhaust stream in response to a control signal from an electronic controller. When it is determined that the HC trap has reached its purge temperature threshold, the controller causes the air pump to provide air to the exhaust stream entering the HC trap. In that way, it is assured that sufficient oxygen will be present in the exhaust stream to oxidize the hydrocarbons released from the HC trap. In fact, the additional air provided by the air pump into the exhaust stream results in an abundance of oxygen in the exhaust stream while the HC trap is being purged.
Because of the abundance of oxygen in the exhaust stream when the HC trap is being purged, the inventor hereof has determined that it is beneficial to control the engine air/fuel ratio to the rich side of stoichiometry while the HC trap is being purged. With the engine air/fuel ratio being somewhat rich, the three-way catalyst will more efficiently reduce NOx. Furthermore, because of the abundance of oxygen in the exhaust stream downstream of the three-way catalyst, any HC and CO that break through the three-way catalyst will be oxidized downstream. As a result, the NOx reduction efficiency of the three-way catalyst can be increased without increasing HC/CO emissions into the atmosphere during the time period when the HC trap is being purged.
Therefore, according to the present invention, when it is determined that the temperature of the HC trap has risen to the purge temperature threshold, the controller causes the air pump to air/fuel ratio provided to the engine cylinders to be biased rich of stoichiometry. The controller maintains this rich bias throughout the period when the HC trap is being purged.